It has been known for a long time that certain amounts of ultraviolet content, in particular, that which is associated with natural and artificial light sources (UV-A 320 to 390 nm; UV-B 280 to 320 nm; UV-C 100 or 200 to 280 nm), lead to damage to the human skin.
UV-A radiation chiefly has the effect of ageing of the skin (thinning of the epidermis and degeneration of connective tissue, and pigment disorders), while UV-B and UV-C lead to sunburn and skin cancer.
Leisure activities which have changed in recent years with longer periods in the open air and, in particular, extensive sunbathing to achieve the “healthy tan” have, however, against the background of medical findings and the awareness of the lack of natural protection mechanisms of the skin by pigment formation and solar acclimatization by thickening of the horny layer, shifted the need for adequate protection against intensive UV radiation. It has been intensified significantly by the discussion of the decrease and thinning of the Antarctic ozone hole and the associated increase in the intensity of UV-A and UV-B radiation on the earth's surface.
This becomes clear from the increasing turnovers in recent years of products with high sun protection factors (SPFs). These are primarily still the conventional sun protection formulations (sun milk, sun oil) with the primary intended use of sunbathing, but increasingly also the so-called care products for the face, body and hair, such as day and night creams, conditioners, lotions, (hydro, lipo) gels, (lip)sticks and sprays, pharmaceutical formulations and to a small extent products of decorative cosmetics, which are predominantly commercially available in the form of oils and liquid, cream-like or ointment/paste-like W/O and O/W emulsions.
As stated above, UV-B radiation can cause tanning and burns, rather than UV-A radiation. Prior art sun protection compositions therefore predominantly comprise only filters which protect against UV-B radiation. Since the effects/side effects of a suntan/sunburn are not immediately and clearly perceptible, the skin is exposed to radiation for significantly longer than would be appropriate.
However, the skin is therefore predominantly exposed unprotected to UV-A radiation. The problem is thus that UV-A radiation penetrates into the skin and causes long-term damage, even though it does not cause immediately detectable actions.
Medical findings in recent years have clearly demonstrated that not only UV-B radiation but also UV-A radiation is harmful to the skin. It has been demonstrated that enzymes which repair cells damaged by UV-B radiation are inhibited in their activity and thus UV-A radiation indirectly promotes skin cancer. By deeper depth of penetration, UV-A radiation can even cause changes in the blood vessels.
Moreover, UV-A radiation is the origin of most photodermatoses, such as sun allergies: small red blisters which appear on the arms and neckline during first exposure to sun and are accompanied by severe itching. In the long term, with abuse of the sun, they can cause skin cancer.
UV-A rays are, therefore, almost more dangerous than UV-B rays, since they issue no alarm signal, such as sunburn. When their damage is noticed, it is already too late.
With increased exposure to the sun, an increase in various infectious skin diseases, such as mycoses or herpes, has been found.
This increase is based on the ability of the UV rays to weaken the immune system of the skin, thus to reduce its capacity to react and to reduce the defense against causes of infections, such as, for example, herpes virus or skin fungi. It is assumed nowadays that the phenomenon of photoimmunosuppression also plays a significant role in the development of skin cancer.
For sun protection of naked, uncovered skin and hair (bleaching, embrittlement), particularly of the face and lips, sun protection compositions, which ensure adequate and lasting protection over the entire harmful UV spectrum, are therefore in demand.
Such broad-band sun protection compositions can comprise a combination of corresponding organic UV-A and UV-B filters.
By these there are to be understood organic substances which are liquid or crystalline at room temperature and which are capable of absorbing ultraviolet rays and of releasing the energy absorbed again in the form of longer-wavelength radiation, e.g., heat. To establish a sufficiently high sun protection factor, however, correspondingly high contents of these filters must be used.
During lasting exposure of the skin to sun, the protection should be renewed at regular intervals of approximately one to four hours. The same applies during sports activity, in order to compensate the decrease in protection, i.e., loss of the filter substances by bathing, perspiration or mechanical abrasion by clothing or hand towels.
Only the regular use of light protection products with a high SPF and a broad absorption spectrum both against UVA and against UVB rays allows an effective protection. Furthermore, because of the increased incident solar radiation in some regions, sun protection formulations, which have a greatly increased light protection factor, are increasingly required.
The light protection factor LPF or also SPF is a coefficient that expresses the ability of a product to prevent sunburn by the sun. Light protection with a factor of 60 therefore protects against the occurrence of sunburn for twice as long as a product with factor 30, and correspondingly 3 times as long as a product with factor 20.
These higher light protection factors are in most cases generated by an increase in the concentration of UV filter substances in the formulation.
Since 1995, light protection factors have been measured by the same international standard (COLIPA), which allows comparison between the various manufacturers.
Given these frequent uses over large areas, it is not ruled out that the high-dosed filters (approximately 3 to 30 wt. % of the formulation) are applied to the skin in gram quantities.
However, these amounts of filter substances must have been dissolved and incorporated into the formulation in a homogeneous and stable manner.
Oily components that have a good dissolving power for the filter substances are often used to dissolve these substances. Certain ester oils, inter alia, are thus also employed. Aliphatic benzoic acid esters are a class of compounds used here. A typical representative of this class of compounds is the compound Tegosoft® TN (C12-15-alkyl benzoate), which has been employed particularly frequently as a solvent for UV filter substances.
Nevertheless, the dissolving power of the established compounds often is not sufficient to dissolve relatively large amounts of UV filter substances.
This increase in concentration is therefore problematic in practice, or under certain circumstances even impossible.
Incompletely dissolved UV filter contents in the end product can put the stated SPF in doubt under certain circumstances. Even if the solution properties of the sun protection filter initially still exist in their entirety, under storage conditions, which are extreme but relevant in practice, precipitation of the filters and, therefore, a loss in action can nevertheless occur.
In view of the above discussion, there is a need to overcome these disadvantages and to provide cosmetic formulations which have a particularly high dissolving power for UV filters.